Today I noticed just in time that the USB flash drive of Rocio, a friend of Esme, had malware on it. After I killed all processes the malware had been able to start on my computer I managed to remove the malware of Rocio's USB flash drive as well.
Next, I used a portable version ClamWin antivirus to scan her USB flash drive, and then copied ClamWin antivirus on her USB drive so she could scan the computer at her house as well.
After that I checked all our own USB flash drives as well, and one that belonged to Esme had some malware on it as well - probably picked up in a shop that printed some files for us - which I also removed.